Impact printers using an ink ribbon to carry the printing medium from which characters on the printed page are formed must employ a ribbon drive mechanism to advance the ink ribbon so that fresh portions of the ribbon are available for printing. The mechanism driving the ribbon must accommodate manual tightening of the ribbon.
FIG. 1 illustrates an impact printer, printer 100, in which the ink ribbon 104 is supplied in a ribbon cartridge, ribbon cartridge 101. Printer 100 also includes a ribbon drive, ribbon drive 102. Ribbon drive 102 may be a ribbon drive according to the prior art, or the ribbon drive of the present invention. A portion (see ribbon pinion 207 in FIG. 2) of ribbon drive 102 engages a spool (not shown) within ribbon cartridge 101 and advances the ink ribbon 104. In order to manually tighten the ribbon 104, ribbon cartridge 101 incorporates knob 103, illustrated in the inset. By manually turning knob 103 in the same direction as the cartridge is driven, the ribbon 104 may be manually tightened. An enlarged view of such a ribbon-tightening knob is shown in FIG. 6. Ribbon drive 102 must accommodate free rotation to permit manual tightening of the ribbon 104 while the ribbon cartridge is engaged with the ribbon drive 102. This is complicated by the large speed reduction that is effected between the speed of the driving means for driving ribbon drive 102, and the speed at which the ribbon 104 advances, which necessarily must be the rotation speed of an output of ribbon drive 102.
Refer now to FIG. 2, in which is depicted a ribbon drive, ribbon drive 200, in accordance with the prior art. Ribbon drive 200 includes ribbon motor 201 and gear mount bottom 202, which supports gear 203 (shown in normal and inverted views), gear 204 (shown in normal and inverted views), and pinion gear 205. These gears are enclosed by gear mount top 206. Ribbon pinion 207 (which engages the spool (not shown) in ribbon cartridge 101) passes through an opening 210 in gear mount top 206 and is coupled to pinion gear 205, through which it is driven. Gear 203, gear 204, and pinion gear 205 effect the speed reduction between the speed of ribbon motor 201 and the speed of ribbon pinion 207 which drives the ribbon 104. Because the speed at which the ribbon 104 advances is substantially slower than the rotational speed of ribbon motor 201, the speed reduction produced through gear 203, gear 204, and pinion gear 205 is also substantial. Consequently, the torque that would need to be applied to knob 103 in order to manually tighten the ribbon 104 would be considerable unless a means were incorporated to decouple ribbon pinion 207 from pinion gear 205. This is achieved in the prior art by the use of spring clutch 208. When ribbon pinion 207 is rotated manually through its engagement with knob 103, spring clutch 208 decouples ribbon pinion 207 and pinion gear 205, and thereby the rest of the ribbon drive 200. In this way, knob 103 and ribbon pinion 207 may be freely rotated, and the ribbon 104 manually tightened, without rotating the remaining portions of ribbon drive 200.
Achieving the required speed reductions through the use of a gear train employing multiple individual gears, and a spring clutch to decouple the gear train from the ribbon pinion increases manufacturing and assembly costs and decreases reliability. Thus, there is a need in the art for an improved ribbon drive that incorporates fewer parts, thereby reducing manufacturing and assembly costs, and improving reliability.